As is known in the art, DC brushless motors may be driven according to different modes, generally depending upon the speed of the motor. For example, a brushless DC motor may be operated in a bipolar mode (i.e., with two coils driven in each phase) during startup. Once a desired operating speed is reached, the motor drive circuitry may switch to a unipolar mode (i.e., only one coil driven in each phase), to reduce the effects of back emf induction in the driven coils at higher rotational speeds. As is known in the art, back emf in a driven coil reduces the available driving voltage that can be applied to the coil, which reduces the drive current to the coil and thus limits the torque of the motor.
Referring now to FIG. 1, a typical arrangement of output driver transistors is illustrated hereof. In this example, which is described in detail in U.S. Pat. No. 5,306,988, issued Apr. 26, 1994, assigned to SGS-Thomson Microelectronics, Inc. and incorporated herein by this reference, a brushless DC motor includes stator (or, alternatively, rotor) coils A, B, C connected to a center tap 6 in the conventional "Y" arrangement. Driver circuit 5 includes, for each of motor coils A, B, C, an n-channel high-side drive, or pull-up, transistor 14 and an n-channel low-side drive, or pull-down, transistor 15. The gates of high-side drive transistors 14, 14', 14" (for motor coils A, B, C, respectively) are driven by lines from a commutation sequencer. The gates of low-side drive transistors 15, 15', 15" (for motor coils A, B, C, respectively), are driven by a respective power switch 7, 7', 7", under control of the commutation sequencer. Sensing resistor 9 is connected between the sources of low-side transistors 15, 15', 15" and ground; the voltage across resistor 9 is amplified by constant gain circuit 8, and applied to error amplifier 10 along with a reference voltage V.sub.IN. The output of error amplifier 10, which is a voltage proportional to the differential voltage at its inputs, is provided to each of power switches 7, 7', 7". During a commutation sequence, the one of low-side transistors 15 that is to be turned on will have the output voltage of error amplifier 10 applied to its gate. As such, the current sensed by resistor 9 determines the amount of low side drive applied by driver circuit 5.
High-side drive transistors 14 and low-side drive transistors 15 in the arrangement of FIG. 1 each include a diode 16 having its anode connected to the transistor source and its cathode connected to the transistor drain. These diodes 16 arise from the body node-to-source connection of transistors 14, 15, resulting in diode 16 present at the p-n junction between the body node and drain for each of transistors 14, 15.
By way of example, FIG. 2 illustrates, in cross-section, the construction of a typical n-channel drive transistor such as high-side drive transistor 14 of FIG. 1. As shown in FIG. 2, transistor 14 includes a source electrode S connected to an n+ source region, a drain electrode D connected to n+ source region 18, and gate electrode 17 overlying a channel region in p-well 11 therebetween. P-well 11 is connected to body electrode B through a surface p+ region and, as shown in FIG. 2, body electrode B is electrically connected to source electrode S, and to node OUTA of FIG. 1. Diode 16 for transistor 14 of FIG. 2 is thus present at the p-n junction between p-well 11 and n+ drain region 18.
In the configuration of FIG. 2, another diode 16' is also present at the p-n junction between buried p+ region 13 (underlying p-well 11) and n-type isolation region 19. N-type isolation region 19 is connected to the positive power supply V.sub.cc via an n+ region and a metal electrode, and serves to isolate p-well 11 from adjacent wells.
The body node-to-source connection of high-side drive transistors 14 provide rapid drive response to commutation in bipolar mode. This rapid response is due to the reduced threshold voltage for transistors 14 relative to that which would be present if body nodes of transistors 14 were biased to ground; in addition, the source/drain "on" resistance of transistors 14 is reduced in this configuration as compared to the case where the body node is biased to ground.
It has been observed, however, that the biasing of the body node to the source of high-side drive transistors 14 is undesirable in unipolar mode, however, due to the presence of diode 16 (and also diode 16' to the isolation regions, as discussed above). This is because the back emf of a coil, when floating in unipolar mode, may rise above the V.sub.cc power supply voltage. For each of coils A, B, C, this condition will forward bias diodes 16, 16' of FIG. 2 associated with its high-side drive transistor 14, 14', 14", respectively, and will short out the motor.
It is therefore an object of the present invention to provide a motor driver circuit in which the driver transistor body nodes may be biased in the most advantageous way in each operating mode.
It is a further object of the present invention to provide such a circuit in which the inductive kick at commutation is clamped.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification, together with its drawings.